Ordinary radiator- move water inlet to top tapping?


Ive got a large, heavy & expensive but otherwise ordinary radiator to which I want to fit a Honeywell "Rondostat" digital TRV.
Sadly it has an LED display that can't be read with the 'stat in the traditional location at the bottom.
The question is if I re- arrange the hot water inlet to use the top tapping will the bottom half of the radiator still get hot ?
DerekG
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wrote:

Diagonal connections will give you the maximum heat output. The manufacturers heat ratings are all for diagonal connections because it makes them look more efficient.
Diagonal connections are the traditional way radiators are connected. Using the two bottom connections is just done to make the pipework look neater. And save a bit of pipe.
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On 14/02/2013 08:52, harry wrote:

Does it make any difference whether the water goes in at the top and out the bottom, or vice versa?
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Google suggests inlet at the top is traditional; with inlet at the bottom you can get a cooler lower corner at the other end as the hot water rises as it comes in.
And it turns out that top and bottom at the same end is actually even better than diagonal (and is used for quoted ratings). But the effects are small in modern systems.
http://www.diybanter.com/uk-diy/326601-radiator-inlet-outlet-positions.html http://www.navitron.org.uk/forum/index.php?topic 116.0 http://www.plumbingpages.com/featurepages/CorrectionFactors.cfm
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On 14/02/2013 09:02, GB wrote:

ICBW but it seems to me it'll be better with hot fed in at the top. That way the top of the radiator will be hottest, and the outlet at the bottom coldest (which you want for boiler efficiency) and you'll also have the advantage of a counter current flow between the air and the water: the air will rise and be coolest where the rad is coolest, and hottest where the rad is hottest.
Andy
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No difference at all. The outlet has to be at the bottom, but the inlet can be anywhere. If you feel a radiator with inlet at the bottom, you will see the hot water rises by convection to the top in the first vertical channel. It makes no difference if this happens inside the radiator or if you pipe it to the top outside the radiator.
The only time the inlet must be at the top is for a gravity fed system (no pump), to generate the dynamic pressure difference in the height of the radiator which generates the gravity flow.
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Andrew Gabriel
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On 14/02/2013 09:50, Andrew Gabriel wrote:

This needs to be clarified as it doesn't fully answer GB's question. I once got a TIBO the wrong way round and I got a hot bottom entry area and hot at the top and everywhere else cold. I cannot stress that if choosing a top connection, it must be on the flow side and the exit must always be a bottom connection.
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Use more lube next time.
MBQ
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On Thursday, February 14, 2013 12:22:17 PM UTC, Fredxx wrote:



Why? How did the water pass from the hot bottom to the hot top, without wa rming the area of the radiator between them?
The head provided by a pump would be many times the head generated by gra vity circulation, so the water will still flow through the rad if connected bottom in top out. I can't imagine why anyone would intentionally do that, but it should still get hot and emit heat.
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gravity circulation, so the water will still flow through the rad if connected bottom in top out. I can't imagine why anyone would intentionally do that, but it should still get hot and emit heat.
The hot water will flow in the bottom, straight up the first channel, and out of the return (passing along the top if it's on the opposite side). The water in the rest of the radiator stays cold, and doesn't circulate, and most of the radiator remains ineffective.
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Andrew Gabriel
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On 14/02/2013 20:07, Onetap wrote:

It warms a little bit of the vertical, not all of it. As FredXX found.
Andy
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On Thursday, February 14, 2013 10:59:59 PM UTC, Andy Champ wrote:

So FredXX says, but I am asking the company to apply their wisdom and say why that should be; since I don't think it would happen unless there are some other factors involved. I have spent a lot of time designing, commssioning and balancing fluid systems. I can't see it happening, unless there's something else happening.
It is plausible with TBSE connections,
It is unlikey with TBOE connections. If you look at a TBOE radiator, there are equal flow distances between the inlet and outlet connections, regardless of which vertical channel the water passes through. You would expect there to be equal flow rates through all of the radiator channels, regardless of which connection was top or bottom.
The things that might cause it to happen, IMHO, are; radiator with some channels obstructed with rust or corrosion products (they often are); second-hand radiator with internal baffles/blanking; radiator airlocked; TBSE connections, especially with a CI sectional radiator (big waterways); CI radiators generally (negligible hydraulic resistance).
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On 15/02/13 13:39, Onetap wrote:

Cold spots will be indicative of restricted flow.
If water has to speed up to bass a bottleneck it will in general dump less energy on the way.
The whole argument is rather academic though, since the total rated power of most radiators is usually far in excess of the boilers ability to supply that power. I.e. the designs are such that most radiators will be throttled back by the balancing valves or TRV's.
So 'radiator efficiency' is only relevant when trying to get the most out of a small area. You aren't wasting fuel by having low 'efficiency' radiators.
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Ineptocracy

(in-ep-toc’-ra-cy) – a system of government where the least capable to
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On 15/02/2013 13:39, Onetap wrote:

Surely the something else is convection isn't it? When the hot, light water goes in at the bottom, it'll float up to the top of the rad, flooding through the top "tank". Flow rates inside the rad due to the pump are low, so convection can dominate and the hot water floats up through the first few channels. If it didn't, BBOE would never work. The cooler water already in the rad will pool at the bottom, where the exit ought to be. If the exit is at the top, then it'll be the hottest water in the rad that gets pushed out by the incoming water - obviously not what you want. A notional BTOE (as opposed to TBOE) rad will only get hot up one end and along the top.

No. I would expect convection to be the driving force in deciding which channels the water flows through. If hot water finds itself at the bottom of a cooler channel, it'll flow strongly up it. With the exit at the top, a large pool of cold water will accumulate at the bottom of the rad.
Cheers,
Colin.
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I've seen this very effect on a system I was balancing for a pal. All rads TBOE but one wouldn't balance correctly and room temp was down. Large pool of cool water from the bottom up was exactly how is manifested and swapping the flow solved it immediately.
Also, whacking more flow through makes little difference, you get a little more mixing but half the rad is still cold so I wouldn't expect more than half output from it and that would be geometry dependent.
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fred
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The factors governing heat exchange are.
Time (spent in the heat exchanger), Turbulence, Temperature (difference)
The more you get of any of them the greater amount of the available amount of energy is transferred per unit of water.
So in fact if the water spends longer in the radiator, it will come out cooler.
However the total heat emitted will be greater as the flow increases. (The temperature difference will be less between inlet and outlet so the average surface temperature will be greater.)
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On 16/02/2013 07:16, harry wrote:

You're forgetting probably the most important one - the effective surface area of the heat exchanger. If the rad has a big cold patch, then its effective surface area for heat exchange is obviously much smaller than it should be.

If the outlet is at the top, then increasing the flow will increase the output power slightly, but then you'll end up with a badly unbalanced system. The rad in question will be a huge flow-hog and the boiler won't be able to achieve its rated output because of all the still-hot water coming back on the return.

The average surface temperature isn't the average of the inlet and outlet temperatures. With the outlet at the top, it'll be well below both of them.
Cheers,
Colin.
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On Friday, February 15, 2013 6:20:36 PM UTC, Colin Stamp wrote:

,

I still don't agree with that. Please note that I'm not saying it won't h appen, I'm saying it wouldn't anticipate it happening.
The convection just generates a pressure difference due to the different masses of the H&C water columns.
With BBOE and no convection (air, rad, water at the same temperature) wat er will still flow up the first channels and down the last, the flow rate b eing such that the frictional pressure loss equals the pressure difference across the inlet & outlet provided by the pump.
With BBOE plus convection, it works as above, but pump and convection for ces work together.
With TBOE (wrongly connected, flow in bottom & return out opposite top) t he pump and convection forces are opposing one another. The pump will IMHO overwhelm the convection forces and ther will be little difference. It will be a parallel flow heat exchanger and the air movement will be reduced.
Domestic hot water cylinders are connected F in bottom, R out the top, bu t no-one has ever claimed it stops them working.
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will still flow up the first channels and down the last, the flow rate being such that the frictional pressure loss equals the pressure difference across the inlet & outlet provided by the pump.

Convection forces are very important. The hot water rises instantly in the first channel, and the rest of the radiator behaves as TBOE, with the hot water descending uniformly down all the other channels, as it cools, and remains stratified almost equally across them all. See
http://www.paceplumbing.co.uk/images/thermal-image-radiator-after-flushing.png

pump and convection forces are opposing one another. The pump will IMHO overwhelm the convection forces and ther will be little difference. It will be a parallel flow heat exchanger and the air movement will be reduced.

The heating coil isn't. But neither are they much of an analogy to a radiator. If you are refering to the outer cylinder, then that's the same as the air convecting up a radiator, with the cold air going in the bottom, and the hot air coming out the top, again by convection.
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Andrew Gabriel
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On Saturday, February 16, 2013 4:01:07 PM UTC, Andrew Gabriel wrote:

g.png
I'm starting to change my mind on this. That image does seem to show all the hot water (white area) going straight up the first channel. Presumably the waterways are oversized (relative to a pipe) for the flow rate that is going through them.
I'm not wholly convinced since the 'before' picture on that site does not show what I'd expect from a sludged-up radiator. There are numerous therma l images of radiators on the internet, but none as 'black & white' as this one; they've probably chosen an image that best suited their sales patter.
TBOE connections would seem to be a waste of time unless you want the val ves high up. The extra heat is probably due to the extra pipe.
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